AP Biology Unit 7 Natural Selection

Genetic Variation

This describes the genotypic and phenotypic differences bw individuals in a population.

• BLANK can lead to different phenotypes and/or adaptations

• BLANK increases the chance a population of organisms will survive under changing environmental conditions.

Genotype

This is the set of genes in DNA responsible for unique traits or characteristics.

Phenotype

This is the physical appearance or characteristic of an organism.

Selective Pressure

This refers to any biotic or abiotic factors influencing survivability.

Ex. disease, predation, climate, food availability

Gene Flow

This is the transfer of genetic material (genes) from one population to another, often through migration or interbreeding, which can introduce new alleles and increase genetic diversity

Genetic Drift

This is a mechanism of evolution in which allele frequencies of a population change over generations due to chance (sampling error). Its effects are greatest in small populations. It can also have major effects when a population is sharply reduced by a natural disaster (bottleneck effect) or when a colony splits off from the main population (founder effect).

It may result in the loss of some alleles and the fixation (100% frequency) of others.

Bottleneck Effect

This occurs when a population's size is reduced for at least one generation due to environmental effects such as famines, earthquakes, floods, fire, disease, drought, etc. It can contribute to genetic drift.

Founder Effect

This is an example of genetic drift that occurs when a small group of individuals breaks off from a larger population to establish a colony. The new colony is isolated from the original population, and the founding individuals may not represent the full genetic diversity of the original population.

Hardy Weinberg Equation

This states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.

p² + 2pq + q² = 1

p + q = 1

p² is the frequency of homozygous dominant, q² is the frequency of homozygous recessive, and 2pq is the frequency of heterozygous. So the frequency of a single recessive gene alone would be sqrt(q²) = q.

The conditions that must be met for this to hold true are:

(1) a large population size, (2) an absence of migration, (3) no net mutations, (4) random mating, and (5) an absence of selection in the population.

Biological Species Concept

This states that a species is a group of organisms that can potentially interbreed, or mate, with one another to produce viable, fertile offspring.

ex. horses and donkeys are different species, so when a female horse and male donkey mate, they produce mules that are generally infertile.

Prezygotic Barriers

These prevent members of different species from mating to produce a zygote, a single-celled embryo.

Examples include:

• habitat isolation: the animals are unlikely to encounter each other

• temporal isolation: the species might reproduce at different times of the day or year

• behavioral isolation: Two species might have different courtship behaviors or mate preferences and thus find each other "unattractive".

• Gametic Isolation: Two species might produce egg and sperm cells that can't combine in fertilization, even if they meet up through mating.

• Mechanical Isolation: Two species might have bodies or reproductive structures that simply don't fit together.

Postzygotic Barriers

These keep hybrid zygotes—one-celled embryos with parents of two different species—from developing into healthy, fertile adults. The barriers are often related to the hybrid embryo’s mixed set of chromosomes, which ay not match up correctly or carry an incomplete set of information.

Speciation

This is the evolutionary process where populations diverge and become distinct species which can no longer interbreed with one another.

There are two types:

• Allopatric: involves geographic separation of populations from a parent species and subsequent evolution.

• Sympatric: involves speciation occurring within a parent species remaining in one location.

Allopatric Speciation

This is when organisms of an ancestral species evolve into two or more descendant species after a period of physical separation caused by a geographic barrier, such as a mountain range, rockslide, or river. Once the groups are reproductively isolated, they may undergo genetic divergence. That is, they may gradually become more and more different in their genetic makeup and heritable features over many generations.

Sympatric Speciation

This is when organisms from the same ancestral species become reproductively isolated and diverge without any physical separation.

Polyploidy (having more than two full sets of chromosomes) can lead to speciation in plants when triploid and diploid plants cannot produce fertile offspring.

Miller Urey Experiment

This was conducted in 1952, and it simulated the conditions of early Earth's atmosphere and demonstrated the possibility of organic molecules, including amino acids, forming from inorganic compounds through chemical reactions.

The experiment aimed to test the hypothesis that life could have originated from simple inorganic molecules under conditions similar to those thought to exist on early Earth.

During the experiment, the scientists generated a variety of amino acids by exposing gases in a chamber to an electric spark.

Convergant Evolution

This is the process where unrelated organisms independently evolve similar traits due to adapting to similar environmental pressures or ecological niches. This means that species from different lineages can develop similar characteristics, like the streamlined body shape of sharks and dolphins, even though they are not closely related

Directional Selection

This favors one extreme of a trait over the other, leading to a shift in the population's average phenotype.

Ex. During the Industrial Revolution, darker-colored peppered moths were favored over lighter-colored moths in soot-covered areas because their camouflage was more effective.

Stabilizing Selection

This favors intermediate traits and selects against extreme phenotype. Human birth weight. Babies that are too small or too large have lower survival rates, favoring an average birth weight.

It causes the range of variation to narrow, and the average trait value remains relatively stable.

Disruptive Selection

This favors both extreme phenotypes and selects against the intermediate trait. Some species of finches with beaks of different sizes are better adapted to different food sources, leading to a bimodal distribution of beak size. The population can diverge into two distinct phenotypes, potentially leading to speciation.

Phototropism

The movement of a plant in response to light.

Gravitropism

The movement of a plant in response to gravity

Hydrotropism

The movement of a plant in response to water.

Chemotropism

The movement of a plant in response to a chemical stimulus.

Thigmotropism

The movement of a plant in response to to touch or physical contact.

R-strategists

This is a reproductive strategy that prioritizes rapid reproduction in unstable environments, producing many offspring with little parental care. Examples include insects, bacteria, weeds.

K-strategists

This is a reproductive strategy that focuses on slower reproduction and high parental investment in stable environment.

Ex: humans, elephants, some birds

Secondary Succession

This refers to the ecological process where an existing ecosystem is re-established after a disturbance like a fire, flood, or clear-cutting. Unlike primary succession, which starts on barren land, secondary succession begins with pre-existing soil and some remaining life. This process involves the gradual return of plant and animal life to the disturbed area, often starting with pioneer species and progressing to more complex communities over time.